1. Field of the Invention
The present invention relates to an antenna device for use in mobile phones or the like, and also to a wireless communication apparatus.
2. Description of the Related Art
In recent years, as the size of a wireless communication apparatus, such as a mobile phone, has decreased and density therein has increased, it is becoming necessary that an antenna device be mounted in a small area of a substrate.
However, mounting an antenna device in a small area requires a reduction in the size and thickness of the antenna device, and thus may degrade the antenna characteristics.
Therefore, for example, as disclosed in Japanese Unexamined Patent Application Publication No. 2000-114992, Japanese Unexamined Patent Application Publication No. 2004-023210, Japanese Unexamined Utility Model Registration Application Publication No. 07-020708, and Japanese Unexamined Patent Application Publication No. 2004-128605, various types of antenna devices having been made smaller and thinner without degrading the antenna characteristics have been proposed. Additionally, frequency variation techniques and an active antenna integral with an amplifier have been developed.
An antenna device disclosed in Japanese Unexamined Patent Application Publication No. 2000-114992 is an antenna having a loop radiation electrode. By connecting radiation electrodes formed on the upper and lower surfaces of a substrate through a through hole, the entire antenna is formed into a loop. A compact antenna device with improved radio radiation characteristics can thus be achieved.
An antenna device disclosed in Japanese Unexamined Patent Application Publication No. 2004-023210 is a dipole antenna in which two antenna elements are arranged to form a single plane, and power is fed to the two antenna elements in a balanced manner. This contributes to the prevention of noise and the reduced thickness of the antenna device.
An antenna device disclosed in Japanese Unexamined Utility Model Registration Application Publication No. 07-020708 is a coil antenna. The characteristics of a coil antenna largely depend on its thickness (specifically, the diameter of a winding core). In this antenna device, therefore, the coil antenna is inserted into a hole provided in a substrate. This reduces the thickness of the entire antenna device without degrading the antenna characteristics.
An antenna device disclosed in Japanese Unexamined Patent Application Publication No. 2004-128605 is a quarter-wavelength patch antenna or an inverted F antenna. The characteristics of such an antenna are largely influenced by the distance from a ground surface of a substrate to a radiation electrode. Therefore, in this antenna device, the radiation electrode of the antenna is extended from the upper side to the underside of the substrate at an end thereof. This reduces the thickness of the entire antenna device without degrading the antenna characteristics.
Other antenna devices similar to those described above are disclosed in Japanese Unexamined Patent Application Publication No. 08-023218 and Japanese Unexamined Patent Application Publication No. 2004-165770.
However, known antenna devices described above have the following problems.
Since the antenna device disclosed in Japanese Unexamined Patent Application Publication No. 2000-114992 is a loop antenna, a larger loop diameter increases dead space. Moreover, since the loop antenna is composed of a radiation electrode formed on the upper and lower surfaces of the substrate, the dead space extends not only over one surface but also over both surfaces of the substrate. This creates dead space that is double or more than double the normal amount. Furthermore, if the design of, for example, a housing of a wireless communication apparatus is altered, the radiation electrode of the antenna needs to be totally redesigned.
The antenna device disclosed in Japanese Unexamined Patent Application Publication No. 2004-023210 is a dipole antenna in which two antenna elements are arranged to form a single plane. Although the thickness of the device can be reduced in this case, it is not possible to reduce the size of the entire device. Moreover, since alignment including the balancing of feeding parts in the antenna device is very complicated, design work for the alignment takes a long time.
To produce an antenna device disclosed in Japanese Unexamined Utility Model Registration Application Publication No. 07-020708 or Japanese Unexamined Patent Application Publication No. 2004-128605, it is required that a coil antenna be inserted into a hole provided in a substrate or a radiation electrode be extended from the upper side to the underside of a substrate at an end thereof. This involves difficult alignment of both configurations and antenna characteristics.
Japanese Unexamined Patent Application Publication No. 2000-114992, Japanese Unexamined Patent Application Publication No. 2004-023210, Japanese Unexamined Utility Model Registration Application Publication No. 07-020708, and Japanese Unexamined Patent Application Publication No. 2004-128605 are discussed on the assumption that the disclosed antennas are single resonance antennas. Therefore, if a multiple-resonance antenna device or a frequency-variable antenna device is produced with any one of the techniques described above, dead space that is double or more than double the normal amount is created or the size of the antenna device increases. In other words, it is virtually impossible to incorporate such an antenna device into a wireless communication apparatus, where compactness and high board density are required. Similar problems arise in the antenna devices disclosed in Japanese Unexamined Patent Application Publication No. 08-023218 and Japanese Unexamined Patent Application Publication No. 2004-165770.